On the mechanism of protein-templated gold nanoparticle synthesis: Protein organization, controlled gold sequestration, and unexpected reaction products.

Abstract

Emerging applications that exploit the properties of nanoparticles for biotechnology require that the nanoparticles be biocompatible or support biological recognition. These types of particles can be produced through syntheses that involve biologically relevant molecules (proteins or natural extracts, for example). Many of the protocols that rely on these molecules are performed without a clear understanding of the mechanism by which the materials are produced. We describe a single-pot reaction in which protein-templated gold nanoparticles (AuNPs) are produced as either solution-suspended colloids or as colloids formed within a solid, fibrous protein structure. We have investigated the mechanism for this process by detailing the reaction kinetics and outcomes through the use of 7 different proteins over a range of concentrations and temperatures. The key factor that controls the synthetic outcome (colloid or fiber) is the concentration of the protein relative to the gold concentration. We find that the observed fibrous structures are more likely to form at low protein concentrations and when hydrophilic proteins are used. An analysis of the reaction kinetics shows that AuNP formation occurs faster at lower protein (fiber-forming) concentrations than at higher protein (colloid-forming) concentrations. These results contradict expectations for reaction kinetics and protein-fiber formation, highlighting the need for a better understanding of the mechanism by which biomolecules can facilitate nanoparticle synthesis. As the protein properties that influence this mechanism are better recognized, researchers will be able to better utilize proteins to generate geometry-controlled AuNPs.